The present invention relates to an output compensating device for compensating for variations in pixel outputs of an image sensor.
In a conventional MOS type image sensor, a light sensor circuit used as a unit pixel comprises, as shown in FIG. 1, a photo-diode PD operating as a photoelectric converting element for producing a sensor current proportional to the quantity of incident light Ls falling thereon, a transistor Q1 having a logarithmic output characteristic in a weak inverse state for converting the sensor current produced in the photodiode into a voltage signal Vpd, a transistor Q2 for amplifying the voltage signal Vpd and a transistor Q3 for outputting a sensor signal Vo in accordance with a timing pulse of a readout signal Vs and which circuit is characterized by its a wide dynamic range obtained by giving the output a logarithmic characteristic, thereby achieving the high sensitivity of detecting a light signal.
The above-described light sensor circuit of the image sensor can produce a sensor current in the transistor Q1 while a sufficient quantity of light Ls is falling on the photodiode PD and therefore detect a light signal at a response speed sufficient not to produce an afterimage of the pixel owing to a relatively small value of resistance of the transistor Q1. However, since the transistor Q1 is set to operate with resistance increased by one order when a current therein decreases by one order, decreasing the current flowing in the transistor Q1 with a decrease in the quantity of incident light Ls falling on the photodiode PD causes the transistor Q1 to rapidly increase its resistance. At the same time, a time constant of the circuit containing a parasitic capacitor C of the photodiode PD with the increased resistance is increased to elongate time necessary for removing electric charge accumulated in the parasitic capacitor C. Consequently, an afterimage can be viewed for a longer duration as the quantity of incident light Ls decreases.
FIG. 6 shows a characteristic change of the voltage signal Vpd when the sensor current of the photodiode PD rapidly changed from IE-10A to IE-15A. As is apparent from the characteristic of FIG. 6, if a pixel signal Vo is output at an interval of 1/30 seconds at a small sensor current of 1E-12A with a small quantity of light falling on the photodiode PD, the voltage signal Vpd cannot be saturated within the above time interval. Thus, the saturation time of a voltage signal Vpd corresponding to a sensor current in the photodiode PD with a decreased quantity of incident light thereto also increases. Therefore, if a pixel signal Vo is output in accordance with timing pulses of a reading signal Vs as shown in FIG. 8, then an output appears with an afterglow that may be of a higher level at an earlier time. In FIG. 8, Vpd′ designates an inverse amplified voltage signal produced by the amplifying transistor Q2.
Japanese Laid-Open Patent Publication No. 2000-329616 discloses a conventional MOS transistor type image sensor composed of a number of a light sensor circuits arranged to form a matrix of pixels, each of which can produce in a photoelectric converting element a sensor current proportional to the quantity of incident light when the image sensor is taking a picture and convert the sensor current into a voltage signal by a MOS type transistor having a logarithmic output characteristic in a weak inverse state and which is also capable of initializing itself before detecting a pixel signal Vo by removing electric charge accumulated in a parasitic capacitor C of the photodiode PD by changing a drain voltage VD of the transistor Q1 to a level lower than a normal for a specified period. This enables the light sensor circuit to immediately obtain a voltage signal corresponding to the quantity of incident light Ls at that time even if a sensor current rapidly changed. Thus, the light sensor circuit may not cause an afterglow of the pixel even with a small quantity of incident light Ls.
As shown in FIG. 3, the above-described light sensor circuit may output a signal with a logarithmic characteristic at a normal quantity of sensor current corresponding to a normal quantity of incident light falling on a photodiode but it may not maintain the logarithmic output characteristic and may have a substantially linear output characteristic at a decreased sensor current due to a delay of charging the parasitic capacitor of the photodiode. In FIG. 3, WA represents a region of responding with a non-logarithmic characteristic output and WB represents a region of responding with a logarithmic characteristic output.
In case of applying an image sensor using the above-described light sensor circuits for taking an image of, for example, a white divisional line indicated on a road ahead of a vehicle running thereon by using an automatic running control system, it can always provide a high quality road image clearly indicating the white line on the road owing to its wide dynamic range enough to take the image of the subject under severe conditions, e.g., with a great change in brightness of the subject while taking video day and night or when taking video from an entrance or an exit of a tunnel. However, the image sensor using light sensor circuits each presenting a unit pixel and having a logarithmic output characteristic cannot effectively use its wide dynamic range if an output range of each sensor (pixel) signal is narrower than a viewable screen area DA for displaying an image taken by the image sensor as shown in FIG. 9.
On the other hand, a constant difference of brightness between a white line and a road is maintained day and night, sensor signals detected by respective light sensor circuits can represent a constant difference of luminosity and can be easily processed by logarithmic operation.
The light sensor circuit having the logarithmic output characteristic and provided with a means for initializing itself for preventing the occurrence of an afterimage may not exhibit its logarithmic output characteristic at a small sensor current produced in a photodiode PD in accordance with a small quantity of light falling thereon (with low level of illumination) as shown in FIG. 3 because of a delay in charging a parasitic capacitor of the photodiode.
The image sensor thus constructed, specifically, using light sensor circuits possessing a logarithmic output characteristic involves such a problem that the image sensor may not effectively use its wide dynamic range if an output range of each sensor (pixel) signal is narrower than a display screen area DA for displaying an image taken by the image sensor.
In an image sensor using light sensor circuits each of which represents a unit pixel and works in such a manner that, when taking an image by the image sensor, sensor current corresponding to a quantity of incident light is produced and converted by a photoelectric element into an electric voltage having a logarithmic characteristic in a weak inverse state using the sub-threshold region property of the transistor and, then, a sensor signal corresponding to the voltage signal is produced and output while initializing itself before detecting a pixel signal by removing electric charge accumulated in a parasitic capacitor C of the photodiode PD by changing a drain voltage of the transistor to a level lower than a normal for a specified period, there is still a problem that each light sensor circuit may loose its logarithmic output characteristic at a low level of illumination.